A magnetic sensor device is a sensor device for detecting magnetic patterns in a detectable body using a magnetic resistance effect element having the property that the resistance value changes under an impressed magnetic field. Because the degree of magnetization in a magnetic pattern possessed by paper money or other detectable body is low the change in resistance value of the magnetic resistance effect element is low. Accordingly, in a magnetic sensor device that uses bridge-connected magnetic resistance elements, the midpoint voltage produced from the midpoint of the bridge-connected magnetic resistance effect elements is low, so amplifying with a large gain such as 1000, for example, is necessary.
The midpoint voltage produced from the midpoint of the bridge-connected magnetic resistance effect elements fluctuates due to variations in individual magnetic resistance effect elements, fluctuations in power source voltage and also fluctuations in temperature. Consequently, then the midpoint voltage is amplified with a large gain, saturation of the amplifier can occur so a correct detected waveform is not obtained. With the art disclosed in Patent Literature 1 and 2, only the changed portion is amplified, omitting the direct-current portion. In addition, with the art disclosed in Patent Literature 3 through 6, only the changed portion is amplified, omitting the offset voltage.